A controlled source electromagnetic (CSEM) survey technique can be used to perform a survey of a subterranean structure for identifying elements of interest, such as hydrocarbon-bearing reservoirs, gas injection zones, thin carbonate or salt layers, fresh-water aquifers, and so forth. With CSEM surveying, an electromagnetic (EM) transmitter is used to generate EM signals. In the marine survey context, EM receivers can be deployed on a water bottom surface, such as a sea floor, within an area of interest to make measurements from which information about the subterranean structure can be derived. The EM receivers may include a number of sensors for detecting any combination of electric fields, electric currents, and magnetic fields.
In some implementations of CSEM surveying, a polarized electric or magnetic dipole source is used to excite an EM signal which propagates through the subterranean structure and is perturbed by geologic variations to depths of several kilometers. Spatially-distributed, multi-component, broadband EM receivers can record the EM energy that has been perturbed by the geologic variations.
Subsurface electrical resistivity anisotropy can affect the accuracy of measurements recorded using conventional CSEM surveying techniques. Subsurface electrical resistivity anisotropy is an effect that causes resistivity in the subsurface to be strongly dependent upon the direction of electrical current flow used to make measurements. The most dominant form of electrical resistivity anisotropy is vertical transverse isotropy (VTI), in which the vertical electrical resistivity can be much larger than the horizontal electrical resistivity.
One traditional technique of determining electrical resistivity anisotropy is by recording inline and offline (i.e., broadside) component electrical and magnetic field data. Inline data refers to data acquired when the EM receiver dipole is aligned along the same axis as the EM source dipole (in other words, the dipoles of the source and receiver are co-axial) and both are aligned with the tow direction. Offline (or broadside) data refers to data acquired when the EM receiver dipole axis is parallel to the EM source dipole axis, but located off axis (dipoles are co-planar), while both axes of the source dipole and receiver dipole are aligned parallel to the tow direction. Having to obtain EM data in both the inline and offline directions means that one or more EM sources would have to be towed first in the inline direction and then in the offline direction, which is time consuming and adds complexity to the surveying technique.